Although endemic malaria has disappeared from the United States, malaria continues to be one of the most important infectious diseases in the world as it kills millions of people each year in countries throughout Africa, Asia and Latin America. The characteristic presentation of malaria is chills followed by a fever ranging from 104-107° F., followed by profuse sweating. Other manifestations of malaria include anemia, decreased blood flow to vital organs, thrombocytopenia, and glomerulonephritis. Additionally, when the central nervous system is involved, symptoms include delirium, convulsions, paralysis, coma, and even rapid death.
Malarial diseases in humans are caused by four species of the Plasmodium parasite: Plasmodium falciparum (Pf), Plasmodium vivax (Pm), Plasmodium ovali (Po), and Plasmodium malariae (Pm). Each of these species is transmitted to the human via a female Anopheles mosquito that transmits Plasmodium parasites, or sporozoites. Once the sporozoites enter the bloodstream of the human, they localize in liver cells, or hepatocytes. One to two weeks later, the infected hepatocytes rupture and release mature parasites, or merozoites. The merozoites then begin the erythrocytic phase of malaria by attaching to and invading red blood cells, or erythrocytes.
The invasion of the erythrocytes by the malarial parasites is the direct cause of malarial pathogenesis and pathology. The fever, anemia, circulatory changes, and immunopathologic phenomena characteristic of malaria are largely the result of red cell rupture and the host's immune response to parasitized erythrocytes. For these reasons, the erythrocytic stage of the Plasmodium life cycle is of vital importance to vaccine development and treatment of malaria.
There are a number of strategies for developing new or novel therapeutics for the erythrocytic stage of malaria. One strategy is to identify parasitic molecules that are critical to the survival of the parasite. Extracellular merozoites released from infected hepatocytes or from infected erythrocytes must invade other erythrocytes within minutes if they are to survive. Invasion by the malaria parasite is dependent upon the binding of parasite proteins to receptors on the erythrocyte surface (Hadley et al., 1986).
Interestingly, different parasite species use different erythrocytic receptors for invasion of erythrocytes. P. falciparum invades erythrocytes through a 175 kDa erythrocyte binding protein called EBA-175. The gene encoding EBA-175 of Pf has been cloned and sequenced (Sim et al., 1990; Fang et al., 1991). EBA-175 functions as an erythrocyte invasion ligand that binds to its receptor, glycophorin A, on erythrocytes during invasion (Camus and Hadley, 1985; Sim et al., 1990; Orlandi et al., 1992; Sim et al., 1994b). In contrast, the human P. vivax and the simian P. knowlesi invade erythrocytes by binding Duffy blood group antigens present on some erythrocytes (Miller et al., 1975). The genes encoding the Duffy antigen binding proteins of P. vivax and P. knowlesi have been cloned and sequenced (Fang et al., 1991 and Adams et al., 1990, respectively).
Sequencing of the genes encoding the proteins used by P. vivax and P. knowlesi for erythrocyte invasion demonstrated that these proteins are members of the same gene family as the genes that encode the EBA-175, the protein used by P. falciparum for erythrocyte invasion (Adams et al., 1992). Homology between the Duffy binding proteins and EBA-175 is restricted to 5′ and 3′ cysteine rich domains. Within these cysteine rich domains, the cysteines and some aromatic residues are conserved, but the intervening amino acid sequences differ. Sim et al. (1994b) demonstrated that the 5′ cysteine rich domain of EBA-175 of P. falciparum contains the receptor binding domain, while Chitnis and Miller (1994) demonstrated that the 5′ cysteine rich region of P. vivax and P. knowlesi contain the Duffy binding domain. See FIG. 1.
What is needed for erythrocytic malaria vaccine development is the identification and targeting of parasite molecules involved in the process of erythrocyte invasion. Blockade of this ligand-receptor-mediated event can inhibit parasite development in vitro.
Invasion of erythrocytes by malaria parasites results from merozoite ligands interacting with erythrocyte receptors. Within Plasmodium a family of merozoite ligands (orthologues) that are erythrocyte-binding proteins (EBPs) has been identified. In the most severe human malaria, Plasmodium falciparum, the EBP is identified as the erythrocyte binding protein-175 (or EBA-175), and in P. vivax, the second most prevalent human malaria and the simian malaria P. knowlesi, the EBPs are termed the Duffy antigen binding proteins (PvDABP or PkDABP, respectively). Analysis of the deduced amino sequence of these orthologous proteins (EBA-175, PvDABP and PkDABP) lead to the classification of seven distinct domains encoding regions of greater or lesser similarity (Adams et al. 1992). Within the molecular family, two regions show significant levels of conservation, these are identified as region II (RII), which encodes for the ligand-binding domain and region VI (RVI), which has as yet an unknown function. EBA-175 RII binds sialic acid residues in conjunction with the peptide backbone of glycophorin A. PvDABP and PkDABP RII bind the Duffy blood group antigens, which are chemokine receptors (Horuk et al. 1993). PvDABP and PkDABP have a single cysteine rich domain (termed F1) while EBA-175 RII contains essentially a duplicate of F1, termed F1 and F2. Both RII and RVI domains contain cysteine rich motifs. One function of RII and RVI is apparently to provide for a conserved tertiary structure, critical for the ligand-receptor interaction that leads to erythrocyte invasion. Erythrocytic invasion by P. vivax merozoites appears dependent on the Duffy blood group antigens while in vitro studies have shown that P. falciparum may use alternative invasive pathways (Narum et al. 2000, Dolan et al. 1994).